Middleware Design Patterns in Node.js

# Middleware Design Patterns in Node.js

Middleware functions are those that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle.

There are several design patterns in Node.js middleware, let's discuss a few of them.

## 1. Serial Middleware Pattern

It's a sequence of middleware functions where each middleware performs some operation and then passes control to the next middleware. Here's an example:

```javascript
app.use((req, res, next) => {
   console.log('Time:', Date.now());
   next();
});
```

## 2. Parallel Middleware Pattern

In this pattern, middleware functions are executed in parallel. This is achieved using `Promise.all` or similar constructs. This pattern is beneficial when multiple independent operations need to be performed.

```javascript
app.use(async (req, res, next) => {
   await Promise.all([middleware1(req, res), middleware2(req, res)]);
   next();
});
```

## 3. Conditional Middleware Pattern

In this pattern, a middleware function is executed based on some conditions. This is useful for cases like authentication, where certain routes should only be accessible if a user is logged in.

```javascript
app.use((req, res, next) => {
   if (req.user.isLoggedIn()) {
      next();
   } else {
      res.redirect('/login');
   }
});
```

## 4. Branching Middleware Pattern

The branching middleware pattern involves redirecting the request to different middleware functions based on conditions. This can be useful for routing.

```javascript
app.use((req, res, next) => {
   if (req.path.startsWith('/admin')) {
      adminMiddleware(req, res, next);
   } else {
      userMiddleware(req, res, next);
   }
});
```

These patterns allow for a more structured and organized way of handling requests and responses in Node.js middleware. They offer flexibility and control over the flow of the application.